The present invention relates to roller clamps for intravenous administration (I.V.) sets and more particularly to an improved roller clamp for I.V. sets having the unique combination of advantages including the benefits of diminishing the effort required to adjust the degree of pinch on the tube and improving the grip on the tube by following the preferred housing dimensional relations as hereinafter set forth as well as reduced manufacturing cost.
Sometime during the period of approximately 1940-50, Intravenous Administration sets evolved from being reusable to being disposable. Earlier, a technique was developed to produce and store medical grade fluids for human infusion, for example, water, as well as dilute solutions of saline, dextrose, and other fluid nutrients as well as certain medicines. These fluids were stored in sealed glass containers and were administered using sterile surgical rubber tubing. The tubing and other administration components were usually re-sterilized and reused. Control of the rate of fluid administration was adjusted by varying the degree of pinch on the rubber tubing. Various pinch clamps were developed for this application.
With increasing usage, the advent of medical grade polyvinyl chloride (PVC) tubing and the need to reduce or eliminate a serious patient/hospital problem of cross-contamination, disposable administration sets were introduced. This solved the cross-contamination problem, but introduced a new one; that of time varying flow rates, i.e., variations from the initial setting of the desired flow rate over time. The pinched plastic tube exhibited the problem of creep or cold flow when pinched and this caused the flow rate to vary with time, usually this variation was large (on the order of 40 percent during the first ten minutes and increasing) and most often was a decrease in flow rate.
The first pinch clamp (which was not parallel acting) to address the time varying flow rate problem was described in about 1967, and a series of improved parallel acting clamps were introduced in the following years, as well as a number of significant refinements. The primary thrust of such newly developed clamps were, prior to 1967, ease of adjustment, and, subsequent to 1967 usually a combination of ease of use and/or adjustable flow rates which also would change little over time.
Another category of improvement was that of a clamp offering a better grip on the tube because if the clamp were accidentally released, the resultant increase in flow, or in the extreme, run away, could be life threatening or deadly.
Manufacturing cost was usually an underlying factor as is the case with most high volume usage products. Typical cost reduction factors available and employed were: (1) make parts small to conserve material, (2) use lower cost materials, and (3) make parts easy to assemble to keep labor costs-down.
Injection molded plastic is the popular approach to producing a disposable clamp. Typically, plastic materials such as Acrylonitrile Butadiene Styrene (ABS) resin or Polypropylene (PS) may be used with a rubber additive, the ABS material generally being more dense than Polypropylene or other plastics which have been used. In accordance with this invention, the preferred material is ABS or PS or PS with the possible addition of Butadiene containing polymer or other rubber like material.
The combination of economic pressures to hold down or reduce costs of health care, the enormous volume of use of infusion sets and the ever increasing cost of plastic resin combine to greatly increase interest in finding a clamp which satisfied the above mentioned need of: (1) ease of use, (2) reduce time varying flow rates and, (3) firmer grip on the tubing, as well as one which permits production at a cost still lower than that obtained by the obvious steps of making the part smaller, increasing the number of production cavities and/or using a lower cost resin.
Roller clamps for intravenous administration infusion sets, sometimes referred to as I.V. sets and typically disposed of, along with the I.V. set, after one use, are well known and are designed primarily to regulate the flow of liquid passing through a soft plastic pliable and deformable tube, usually polyvinyl chloride with a low degree of extractables. The degree of pinch of the tube is normally used to decrease and thereby regulate the flow rate to a desired value, normally measured in drops per minute when utilizing a drip chamber, the latter representing the prescribed rate at which fluid of interest is desired to be intravenously administered. These pinch clamps are also used as an on/off device, that is, either to permit flow or to stop flow. It goes without saying that the pinch clamp should, once set, accurately control the flow rate over time and also be capable of fully shutting off flow when that is desired.
Typical of the prior art clamps are those of the prior Adelberg patents, as follows: U.S. Pat. No. 4,047,694 of 1977; U.S. Pat. No. 4,013,263 of 1977; U.S. Pat. No. 3,685,787 of 1972; U.S. Pat. No. 5,014,962 of 1991; U.S. Pat. No. 4,725,037 of 1988 and U.S. Pat. No. Re 31,584 of 1984 whose disclosures are referred to and incorporated herein by reference.
In general, there have been two basic types of IV set roller pinch clamps, one being the xe2x80x9cinclined rampxe2x80x9d clamp and the other being what is hereby referred to as a xe2x80x9cparallel actingxe2x80x9d clamp or one whose wheel travels in a generally parallel relation to what is generally referred to as a clamping surface. In each case, the clamp basically includes a housing in which is received a wheel (roller) typically supported by the housing with the plastic tube received in the housing and located between a base or clamping surface in the housing and the roller.
Regardless of the type of clamp, in one extreme position of the roller, there is xe2x80x9cfull flowxe2x80x9d, i.e., unregulated flow at the full flow capacity of the I.V. set. In another position of the roller, typically spaced from the full flow position, there is a no flow position in which there is no flow through the tubing. This typically is a shut off position. In some clamps, there is another shut off position, more properly a lock position in which movement of the roller is affirmatively prevented, as will be described, and there is no flow through the tubing. Between the full flow position and the no flow position, there typically exists a region of travel of the roller over which flow may be controlled, i.e., the region of xe2x80x9cflow controlxe2x80x9d so called. The length of the flow control region may be less, in an axial direction, than the maximum distance over which the roller travels, especially if there is a lock position.
In the case of an inclined ramp clamp, flow control is achieved by an xe2x80x9cinclined rampxe2x80x9d principle in which the roller is forced, by the operator""s thumb, to climb a ramp, causing a decrease in the clearance between the roller and the opposed housing surface upon which the tubing rests, thereby creating the desired degree of pinch and thus controlling the flow rate. As the roller or wheel advances, the clearance between the roller and the surface of the housing upon which the tube rests diminishes and, in the extreme position, creates full (pinched tube) shut off. For typical flow rate settings of the clamp, the tube lumen is fully collapsed in the large center region of the tube""s cross-section (where the tube radius of curvature is large), while a pair of lumens form at either side, where the radius of curvature of the pinched tube is smallest and thereby offers the greater resistance to pinch, The phenomenon of cold flow or creep in the plastic tube explains why the newly formed lumens continue to collapse and cause the flow rate to decrease, after the roller is brought to and remains in its new position.
In a parallel acting (dual action) clamp, the effects of creep or cold flow are reduced or, ideally, eliminated by having everywhere in the flow control region of the housing, a section of the housing which guides the wheel such that a tube clearance is formed which causes at any particular tube cross-section, at least one, and usually both, lumen(s) which tend(s) to form at one or both outer edges, to be fully pinched shut. Flow rate is varied by varying the cross-section of the lumen which is formed elsewhere. Fine rate control at any tube cross-section is achieved at that tube cross-section, by varying the ratio of the fully pinched shut portion of the pinched tube to that other portion which remains open, because there is a relief portion in the base or housing or wheel in which no or less, pinching is there applied. The action, or travel, of the roller need not be precisely or even nearly parallel to the opposing pinching surface of the housing. The criterion for a parallel acting clamp is to have a clearance between the roller and the housing which causes the local portion of the tube to be fully pinched shut, and to have the remaining tube portion located opposite a section of the housing surface or in some designs a portion of the outer wheel surface, which contacts or is near the tube forming a relief portion which permits and/or encourages the formation of an open lumen in the tube.
In the parallel acting type of clamp, the roller at a typical station totally pinches closed a portion of the tubing, and usually a relief in the housing is used to bring about the desired degree of pinch or partial closure of the tube. While the term xe2x80x9cparallelxe2x80x9d is used, it is not intended to describe a geometric parallelism or an arrangement in which the roller travel is in a precise parallel fashion over its entire range of travel; and such xe2x80x9cparallelxe2x80x9d action is only needed in the flow control range of roller travel. In fact, a parallel acting clamp may, and usually does, include a small relative angle between the roller travel and the opposed surface upon which the tubing rests and is clamped, for example, as a result of xe2x80x9cdraftxe2x80x9d used in the injection mold design, as will be discussed. However, this relatively small angle is, in itself, insufficient nor is it intended to effect primary control or variation in the rate of flow since the change in clearance between the roller and the surface upon which the tube is clamped is insufficient to directly vary the tube cross-section area of flow, in contrast to an inclined ramp clamp. It is also the case that in the region of the housing before flow control and after the shut-off or no flow position, as contrasted to the lock position, the clamp action may also be non-parallel in its structure. Since these are regions other than where flow control is accomplished, the need for even approximate parallel action, as herein described, is not relevant or necessary.
Although a parallel acting roller clamp design in which only one cheek of the tube is fully pinched shut may constitute an improvement in clamps designed for good performance with respect to time varying flow rates (flow that varies over time), by fully pinching one lumen that would otherwise be formed and tend to exhibit creep or cold flow, more advanced designs fully pinch shut both cheeks of the pinched soft plastic tube.
The feature which distinguishes a parallel acting (dual action) roller clamp from an inclined ramp clamp is how flow control is achieved. Most parallel acting roller clamps achieve control of flow rate by fully pinching shut one cheek and usually both cheeks of the tube over the entire range of flow control and vary the ratio of fully pinched shut to unpinched or partially pinched, open portion of the tube according to the position of the roller in the region of flow control. It is apparent that for a given tube cross-section, fully pinching shut one or both cheeks, but not the adjacent tube section, is needed only in that range of roller travel which is effectively the flow control range. In the case of an inclined ramp clamp, flow control is achieved by varying the clearance between the roller and the opposed pinching surface. It is thus apparent that even though true parallelism may not exist in a xe2x80x9cparallel actingxe2x80x9d clamp, the amount of non-parallelism is insufficient to vary the clearance by any amount which substantially effects a change in tube lumen or corresponding in flow rate. This is especially true if one or both cheeks are fully pinched shut over the range of travel of the roller in the flow control region.
One typical arrangement to assure that one or both cheeks of the tube are fully pinched shut is to provide a clearance between the bottom of the roller and the opposed surface of the housing against which the tube is pinched which is generally not more than twice the wall thickness of the unpinched tubing and, in many cases, less than twice the tube wall thickness. In general, a clearance of less than twice the wall thickness operates satisfactorily to assure that one or both cheeks are fully pinched shut. Again, it is understood that this defined clearance need only exist in the flow control region. In fact, the clearance is greater at the entry end of the housing and may be somewhat less near the exit end of the housing, especially if there is a separate shut-off lock.
In the xe2x80x9cparallel actingxe2x80x9d clamp, the clearance between the roller and the opposed housing clamping surface may or may not vary with roller position but this variation of clearance with roller position does not substantially control the size of the formed lumen. Control, in this case, is achieved by varying the ratio of clamped shut wall portion of the soft tube to unclamped portion along the control portion of the housing.
In the inclined ramp clamp design, as the roller advances, the center section of the pinched tube may sag and be fully collapsed shut, but the clearance between the roller and the opposed housing surface: (1) is large enough to permit lumens to form, and usually, at one or both cheeks of the tube, and (2) varies to create the desired lumen size(s) and corresponding flow rate.
In a less popular version of a parallel acting clamp, the relief is in the surface of the roller. It is a general characteristic of these types of clamps that the cross sectional open area of the tubing is varied as the roller progresses along its travel, the amount of open area being related to the configuration of the relief of the clamping surface. As is known, the relief may take a wide variety of configurations and may be off to one side of the housing, or beneath the tubing or some other location usually being of varying width and or depth or both. Its function is to vary the tube open area of flow in response to movement of the roller. In effect, varying the lumen formed operates to vary the flow rate and the various parallel acting devices have this common characteristic. For purposes of this invention, xe2x80x9cparallel actingxe2x80x9d is intended to mean a pinch clamp having a roller and a housing and means to vary the tube lumen formed to vary the flow rate and in which there is some form of relief in the roller surface or housing or clamping surface and wherein the roller travels in a parallel relation to the clamping surface, parallel travel being as already explained.
In virtually all models and types of pinch clamps using a roller and a housing, the axle of the roller bears against a guiding surface as the wheel or roller position is altered as well as when it is in place creating the desired degree of pinch.
In the pinch clamps to which this invention relates, there are special requirements, and some of the important ones are as follows: (1) The upper surfaces of the housing should be designed to resist compression by the contacting roller axle. (2) The upper surface of the housing should be designed to resist flexure as well as outward rotation when subjected to an upward force such as that caused by the axle of a pinching wheel. (3) The side walls of the housing should be designed to withstand tension. (4) The lower surface of the housing should be designed to resist flexure as well as operate in compression, provide for a variable width relief action as well as an ideal location for tailored raised elements. The raised elements, if employed, have varying height, width and spacing. (5) The housing should be made of a plastic which can be injection molded, should have a cross-sectional area designed to facilitate flow of the injected molded plastic to fill the entire steel mold cavity and provide the rigidity needed for mold ejection, subsequent assembly, and normal use.
When parallel acting clamps were introduced to the market in commercial volumes in about 1975, the housings measured about two inches in length. The competing inclined ramp clamp housings measured about 1.4 inches in length. The parallel acting clamps captured a significant share of the market due to their better performance as compared to inclined ramp clamps and, by 1978, accounted for essentially all of the sales by one (Abbott Labs) of the two (Baxter Travenol is the other) largest firms in the United States involved with products requiring clamps for use in I.V. sets. In about 1983, Baxter started to use a patented longer clamp. Thereafter, others such as Cutter (which actually started at an earlier date), McGaw, IVAC and Borla, introduced clamps with two inch housings and a trend in clamp length was thus established.
With the more widespread use of the longer clamps, the manufacturing costs increased substantially for a large portion of the market, even though many of the longer clamps did not offer the improved performance of the patented parallel acting clamp. Some of these companies merely followed the trend to a longer housing length. In the case of the parallel acting clamps of Abbott, there was a functional reason for the length, in addition to providing non-time varying flow rate, and a firmer grip on the tubing, the longer roller travel permitted finer adjustment of the degree of pinch and thus the flow rate.
Hence, in an effort to conform to the trend to greater length, the market introduced heavier clamps, apparently showing little concern for, or giving lower priority to the consequent increase in manufacturing costs of these relatively heavier clamps. These cost increases represent a substantial sum given the number of clamps used on an annual basis.
Recently, the cost of producing a roller pinch clamp, and many other plastic items, has dramatically increased from a plastic resin cost portion being a fraction of 50 percent of the production cost to currently where the plastic resin cost constitutes a large or dominant portion of the manufacturing cost. Currently, the molding cost fraction is relatively small, and the raw material cost fraction is relatively large and growing. Thus, if the moldable, useable part can be re-designed to provide a function at least as good but utilizing significantly less plastic resin, a large savings in production costs may be realized.
Most forms of clamps have a housing whose wall thickness cross-section is fairly uniform throughout. Such a design parameter is simple to specify during the clamp""s design by first focusing on the important inner surface configurations: housing pinching surface, wheel axle guide, width of wheel slot etc., and then completing the design by simply specifying a uniform wall thickness throughout.
The new form of clamp of this invention not only provides the desired type of pinch (if parallel acting, the side walls of the tube fully pinched shut etc.) but, in addition, incorporates strength (flex-modulus, etc.), and mold design (injected plastic which completely fill passages) and has a structure to achieve a substantial reduction in the material required for each clamp.
Presently, typical roller clamps have dimensions in the following ranges:
(1) Wheel major diameter width of 1.3 to 1.8 the nominal tube outside diameter, and
(2) Housing wall thickness in the range of two and generally to three and greater times the undeformed soft plastic tube wall thickness, and
(3) Uniform wall thickness over the entire cross-section and such cross-section being the same over the operational length of the housing where flow adjustment occurs (the region of flow control).
An obvious approach to reducing the part weight is to reduce the length of the housing. However, this is not desirable because one portion of the housing length, usually at its large end is needed to facilitate introduction of the wheel so as to make the combination operable. The remaining portion primarily serves to provide a reasonable travel range for the wheel so as to gradually pinch the tube at any position along the relief or control section of the housing.
It is not practical to decrease the overall or major width or height of the clamp housing cross-section because if done so, it will be difficult to surround and pinch a given diameter of soft plastic tube.
Present pinch clamp housings have a uniform wall thickness in the range of 0.3 to 0.5 times the O.D. of the tube for which the clamp is intended to pinch.
There is a practical minimum housing length. For popular size tubing having an outer diameter in the range of 3xc2xc to 4xc2xd mm, the minimum length of the housing is in the range of 23 mm to 55 mm, or approximately 7 to 25 times the tube O.D. The wall thickness of the clamp housing is typically 1.2 mm to 1.7 mm or about 0.2 to 0.5 times the tube O.D. Most importantly, the clamp housing is usually uniform in wall cross-section thickness. This is apparently done (that of uniform housing wall thickness) because:
1. It is simple to specify.
2. It is a conventional design parameter for injection molded parts. Uniform wall thickness facilitates filling of the steel mold by the injected plastic.
3. Most clamp designs give little, consideration to variations in expected mechanical deflection of the plastic housing when subjected to the stresses associated with tube clamping.
Mechanical deflection (strain) depends upon many parameters and for a given plastic resin material, say ABS (acrylonitrile butadiene styrene), which is the preferred material in accordance with this invention, is dependent, in part, upon the level of stress and the type of stress (tension, bending, compression, torsion etc.). A good design will minimize the deflections when under load, or will, by design, allow for deflection but such deflection is to be within the elastic limit, or, if the elastic limit is exceeded, minimize the amount of deflection or degree of excess beyond the elastic limit.
When a clamp housing is put into use, i.e., assembled with a wheel and a pinched tube and the other components of an I.V. set, the expected deflections of the clamp take place, and the amount depends, in part, upon the type (tension, bending etc.) of the stress which is applied. The side walls of the clamp housing are primarily in tension while the top edge or top wall is subjected primarily to bending or flexing, with a secondary component of local compression, due to the pressure applied by the roller wheel axle where it contacts this upper surface. In the case of an injection molded pinch clamp having one or more housing wall sections in tension, according to this invention, this requires a smaller cross-section as compared with the top or bottom walls which are not primarily in tension. The distortion, or mechanical deflection of the housing, when used to pinch the tube, will be due to:
1. Primarily flexing or bending on at least two modes)of the top edge or wall of the housing.
2. Secondarily due to tension of the housing side wall.
3. To a still lesser extent, due to the local compressive force exerted directly by the wheel axle upon the upper edge of the housing where contact is made.
4. Pressure applied by the major circumference of the wheel against the lower portion of the housing surface, through the double wall thickness of the pinched soft plastic tube.
Now, if one looks to prior patents related to clamps, most patents make no written mention of side walls being thinner than the top or bottom walls of the housing and the top wall being thicker in cross-section than the bottom wall. Further the drawings of some of the patents relating to roller clamps tend to describe a variety of configurations, but offer no teaching as to why wall thickness dimensions are as illustrated. For example, U.S. Pat. Nos. 1,411,731; 1,959,074; 2,595,511; 3,135,259; 3,189,038 and 3,289,999 seem to illustrate uniform wall thickness structures. U.S. Pat. Nos. 3,099,429; 3,215,394; 3,215,395; 3,297,558 and 4,340,201 include drawings which could be interpreted as showing thin walled structure. Of these, U.S. Pat. No. 3,099,429 issued to Broman deserves special comment.
Broman illustrates a clamp with thinner top and side walls and the clamping surface or bottom wall appears to be thicker than the side walls. In Broman, the guide surface is the same thickness as the thin side wall and much thinner than the clamping surface. Broman makes no mention of weight savings, strength considerations (flexure, tension etc.). No mention is made as to why the wall thickness was as illustrated and it would appear that the cross-section thickness at the various locations was merely an arbitrary illustration.
When injection molded housings, used for roller clamps, are put to use, their side walls are generally in tension. While their top and bottom walls are exposed to a flexure or compression load because these walls in tension carry higher loads, the wall thickness needed for good design is generally thinner for the side walls. It is interesting that none of the patents cited make any mention of thinner side walls. These patents include: Dabney et al, U.S. Pat. No. 3,802,463 of 1974; Becker, U.S. Pat. No. 4,340,201 of 1982; Becker, U.S. Pat. No. 4,475,709 of 1984; Karpisek, U.S. Pat. No. 4,869,721 of 1989; Forberg, U.S. Pat. No. 4,895,340 of 1990; Nakada, U.S. Pat. No. 5,190,079 of 1993 and German DT 28 55 572 of 1979.
A German firm named Clinico makes a roller clamp for use in Germany similar to those found on the United States market and with a weight to length ratio of 0.594 grams per cm. That clamp has a wall thickness of some 10% less than that of the U.S. clamps, yet weighs some 22% less, on the average. This weight difference in part, is due to the type of tube holder built into the clamp housing. The tube holder is at the end of the clamp at the open or input end of the clamp. In the Clinico clamp, the tube holder is much smaller than of the U.S. clamps and additionally, it is believed that the Clinico clamp is molded from a resin having a density lower than ABS.
Many commercially available clamp housings have a length in the range of 5.2 to 5.7 cm, are molded of ABS or polystyrene and have a weight to length ratio of between 0.65 to 0.72 gm/cm. By contrast, a clamp of this invention may have a length of 5.6 cm and a weight to length ratio of 0.393 gm/cm, measured in the same manner, but see the discussion below.
It is thus an object of this invention to provide a pinch clamp that is easier to adjust, provides a firm grip on the tubing, whose performance is as good as the presently available commercial clamps, one which includes a top wall which is thicker in cross section than the bottom wall and side walls are thinner than the top walls and preferably thinner than the bottom wall, as well as one which is economical to produce.
The objects of this invention are achieved by the provision of improved roller clamps for I.V. sets and more particularly to an improved roller clamp for I.V. sets having the unique advantages of substantially reduced manufacturing cost and the benefit of good performance by diminishing the effort required to adjust the degree of pinch and improving the grip on the tube when the preferred housing dimensional relationships and the preferred relative dimensions are as hereinafter set forth.
Thus, the preferred and practical approach of this invention is to specify selectively the portions of the cross-section of the clamp housing. A refinement in accordance with this invention is to further vary the cross-section according to its location along the housing length axis. For example, the cross-section at the region of the portion of the housing designed to permit insertion of the wheel, has reduced strength requirements. In this region the wheel axle does not bear forcefully upon the opposing housing surface through a clamping action on the tubing.
Furthermore, there usually is little, or, very often, no pinch action on the tubing in this region. On the other hand in the generally longer operating section of the housing, the region of flow control so called, the structure must anticipate greater forces resulting from the interaction of the wheel, housing, and pinched tube.
Hence taking into account all of the factors discussed in connection with stresses and deflection and the like, a new and unique clamp housing design and new and improved roller clamp for use with I.V. sets, in accordance with this invention, is for the flow control portion, a housing, having:
a. thick cross-section top edge or top wall for the housing.
b. a generally thin side wall housing, as described.
c. a thick bottom wall (but not quite as thick as the top wall or the top edge) of the housing.
Important is the relationship of the ratio of the cross-sectional thickness of the top wall to the cross-sectional thickness of the side wall, which in accordance with this invention is in the range of 1.3 to 4.0.
Note that a thick cross-section top wall or edge as per (a) is used to minimize the degree of at least two types of bending or flexing as such deflection can be large and a thick cross-section will minimize this. A second justification for (a) above is to minimize the degree of local deflection or indentation of the housing surface just where the wheel axle is applying a relatively large, local force.
(b) above is used to reduce the weight of the clamp housing material and to take advantage of the fact that there is a relatively smaller deflection when under tension, and also the fact that thin plastic walls are capable of handling relatively large tensile forces.
(c) above is used to stiffen (but no more than is necessary) the bottom wall to accommodate bending and flexing. Parameter (a) above is designed for the housing to be subjected to a relatively large force causing bending and flexing, which force is applied by the wheel axle. Parameter (c) is designed to respond to a similar opposing force, also causing bending and flexing, but this force is more distributed because it is transmitted and applied by the pinched section of the soft plastic tube, and also because this surface is wider.
The greatest degree of flexing will take place when the wheel is centrally located between, and thereby approximately equally distant from, the end supports of the housing. Thus, the axially centered portion of the upper wall portion should be correspondingly thicker than at either end. However, this is not practical for injection molded parts as a xe2x80x9cdraft anglexe2x80x9d of the part will not be present to permit removal of the just molded part which when conventionally injection molded, will shrink to firmly grasp the mold core. A practical compromise, however, is a design where the wall thickness of the housing is diminished near the small end of the housing, but not diminished as much at the open end of the housing.
Creep or cold flow may take place in the wheel and housing of the pinch clamp (it usually takes place to a greater extent in the pinched soft plastic tubing). To An:exist, the load applied must cause the part to exceed its elastic limit. If creep or cold flow exists, the part dimension may change with time, with no external changes in the load and this will contribute to a time varying flow rate of the fluid through the pinched tube. To avoid this undesirable phenomenon, the load should be limited, or the wall cross-sections sufficiently large (and correspondingly strong) so as to prevent exceeding the elastic limit or minimize the stress. On the other hand, deflections must be anticipated because strain usually accompanies stress. It is usually best to attempt to operate within the elastic limit.
Since the part described in this invention is made of plastic and preferably molded, for proper injection molding, a xe2x80x9cfillxe2x80x9d cross-section or tunnel is needed in the part to assure that the molten plastic resin, usually at a temperature in the range of 350xc2x0 F. to 500xc2x0 F., will readily flow so as to rapidly and entirely fill the cavity. If the tunnel or fill cross-section is too small, the molten plastic will cool too rapidly and freeze prior to there being a complete fill. At lower temperatures, the injected plastic resin""s viscosity, which is very temperature dependent, will be so high that a partial fill may result. Too large a tunnel or fill cross-section will eliminate the partial fill problem but will result in a part using excessive (and costly) plastic resin and also will unnecessarily increase the time (and thereby the production cost) required to cool the injected plastic to a temperature permitting ejection of the formed part from the mold cavity and still be able to retain its newly formed shape (no warping etc.). A long residence time in the mold increases the molding cost, usually in a linear manner. That is, if the cycle time to produce the molded part by the molding machine is doubled, the molding cost is approximately doubled.
Insofar as the molding operation and mold design are concerned, it is important to the present invention to properly select and combine the size and location of the fill cross-section or fill tunnel having the properly designed cross-section as to provide both the needed strength when the part is later called upon to perform by being sufficiently rigid or strong in the bending, flexure or tensile mode as well as to provide a fully formed part. Thus, the preferred form of the mold for proper molding has internal cavity and external core surfaces sized and located and serving the independent requirements of having:
(1) A properly sized and located fill tunnel or fill cross-section adequate but not so large as to unnecessarily increase the part weight and/or slow the production cycle.
(2) A properly sized and located flexure or tensile cross-section to provide the desired strength, rigidity or flexure, just adequate but not too large.
(3) Walls having suitably thin cross-section satisfying other demands of tensile strength and part integrity, yet utilizing the minimum plastic so as to keep the total weight low and also to facilitate a short cooling time for a faster production time. Faster production time makes possible larger production volumes for a given mold time interval and machine, typically a doubling of cross-section increases the molded part cooling time and corresponding cost of molding approximately fourfold.
As explained above, the housing top wall is made thicker so as to have the strength necessary to have the needed flexural rigidity and to prevent excessive penetration (indentation) due to the local force of the wheel axle.
By way of explanation, should the top wall or top rail have a thickness less than a minimum value, the penetration of the wheel axle may be so large as to cause a permanent deformation resulting in time varying flow rates through the pinched tube, due to creep or cold flow of the plastic forming the top rail/wall of the housing. This creep and cold flow results when the plastic used to mold the housing exceeds its elastic limit. Excessive penetration will also make it difficult for the operator to adjust the wheel position. This is because a deeply penetrating axle will require a relatively large force to advance the wheel to a new location. A plastic material with a high value for Rockwell hardness will offer the best resistance to local penetration by the wheel axle, but plastic material cost and moldability may detract from the contribution of a large value of hardness.
In accordance with this invention, the top wall or rail of the housing, when used under load or in use, must withstand three types of deformations. The primary task is to resist flex over the entire length of the housing while applying the desired clamping force upon the roller axle. In addition, as mentioned, the top wall must resist excessive wheel indentation of the wheel axle into the local surface of the top wall, i.e., the top of the trunnion groove. A third consideration is resistance to outward rotation of the top wall leading to xe2x80x9cpop outxe2x80x9d of the wheel, as will be discussed. In each of these cases, it is important that for the top wall not to exceed the appropriate elastic limit.
However, a thick wall section generally requires a longer time to cool, a necessary step in injection molding. This longer cooling time makes for higher manufacturing cost because of longer mold cycling time. All of the above is correct and true where the cooling is one dimensional, such as is the case for a part having a thickness small compared to its other two dimensions.
However, where one other dimension, perpendicular to the thickness dimension, compares with the thickness, the cooling, effectively, is two dimensional and correspondingly requires much less time. Thus, to avoid excessive cooling times, the thick wall top portion should have a thickness comparable to its width. This serves a double function of providing for the more rapid two dimensional cooling as well as providing a generous flow passage for the injected molten plastic facilitating a complete fill of the adjacent thin side wall portion of the housing.
Another aspect of this invention is the relative ease by which currently existing molds may be modified to obtain the benefits of this invention. One feature includes a relatively simple change to the injection molding cavity by bringing the two cavity halves closer together to produce thinner side walls (which must withstand tensile forces in use).
Another feature includes (1) shifting the mold core with respect to the cavity to (a) increase the wheel axle guide portion thickness of the housing (to provide needed flexural strength as well as resistance to penetration by the wheel axle), (b) locate the injection molding gate so as to assure good distribution of molten plastic, (c) significantly reduce the housing clamping surface wall thickness (because of its inherently larger wall cross-sectional area), (d) introduce or retain two dimensional heat transfer (cooling) where the wheel axle guide in the housing has its thicker wall cross-section (so as to keep down production molding cycling time).
In the case of the housing, over the range of flow control, the cross-section of the housing in accordance with this invention is such that:
1. Side wall thickness is in the range of 1.2 to 2.2 times the undeformed (nominal) soft plastic tube wall thickness (and not 2.2 to three times).
2. Top wall thickness (wheel axle guide) is 2 to 3.3 times the undeformed soft plastic tube wall (this is in the typical range or thicker).
3. Bottom wall thickness is preferably less than that of the top wall (as opposed to being generally equal).
4. Side wall thickness of approximately 0.3 to 0.6 that of the top wall thickness (as opposed to being generally equal). As noted earlier, the ratio of the cross-sectional thickness of the top wall to the cross-sectional thickness of the side wall is in the range of 1.3 to 4.0.
Further details are hereinafter set forth. In this way, the objectives noted above are achieved.
This invention has many other advantages, and other objectives, which may be more clearly apparent from consideration of the various forms in which it may be embodied. Certain versions of such forms are shown in the drawings accompanying and forming a part of the present specification. These forms will now be described in detail for the purpose of illustrating the general principles of the invention; but it is understood that such detailed description is not to be taken in a limiting sense.